1. Field of the Invention
This invention relates to an ultrasonic fluid vibrating flowmeter for measuring the flow rate of a fluid; and more particularly, to such a flow meter which is improved to provide stable, substantially noise-free operation.
2. Description of the Prior Art
One type of known fluid vibrating flow meter is the vortex flowmeter which measures the flow rate of a fluid to be measured by using ultrasonic signals to cause vortexes of the fluid (which is one kind of fluid vibration caused when the fluid to be measured contacts a vortex generator) and counting the number of vortexes, whereby the flow rate is measured from the frequency thereof.
Another type of flowmeter is the fluidic flowmeter which measures the flow rate of a fluid to be measured by causing the fluid to be jetted through a nozzle onto a target and then measuring the flow rate by taking the difference of pressures on either side of the jet flow, and by measuring the vibration of the fluid vibration caused by the jet hitting the target by using a piezoelectric sensor located on an inner wall of the pipe carrying the fluid.
In the instant specification, the conventional art will be explained on the basis of the vortex type flowmeter, such as disclosed in Japan UM 48-17010 entitled "Flow Velocity Measuring Device", and illustrated in FIGS. 1-3, wherein FIG. 1 shows the structural features thereof, FIG. 2 shows changes in propagation timing, and FIG. 3 shows components of the flowmeter.
FIG. 1 depicts a conventional vortex type flowmeter wherein a vortex generator 1 is disposed in a fluid flow to generate Karman vortexes, which generator 1 is shown as a columnar object. In FIG. 1 vortexes 2 are generated by vortex generator 1 which is positioned in measuring pipe line 3, through which a fluid to be measured is caused to flow. An ultrasonic receiver 5 and an ultrasonic transmitter 4 are mounted on pipe 3, on the downstream side of generator 1, facing each other and at approximately right angles to the fluid flow direction. These comprise a detector for detecting the number of produced Karman vortexes , that is the number of vortexes which flow per unit time.
If there are no vortexes in the propagation path of the ultrasonic signal, which is shown by the dotted line in FIG. 1, the propagation time .tau..sub.o can be expressed as follows: EQU .tau..sub.o =D/C.sub.A ( 1)
wherein D is the distance between transmitter 4 and receiver 5, and C.sub.A is the speed of sound within the medium (i.e. fluid).
Next, propagation time .tau..sub.1, when Karman vortex exists in this pipe, and when the transmission direction of the ultrasonic signal and the direction of velocity component V.sub.1 of the vortex is the same, may be expressed as follows: EQU .tau..sub.1 = d.sub.v /(C.sub.A +V.sub.1)!+ (D-d.sub.v)/C.sub.A !(2)
wherein D.sub.v is the diameter of the vortex.
Furthermore, the propagation time .tau..sub.2, when the Karman vortex exists in this pipe, and when the transmission direction of the ultrasonic signal and the direction of the velocity component V.sub.2 of the vortex is opposite, may be expressed as follows: EQU .tau..sub.2 = d.sub.v /(C.sub.A -V.sub.2)!+ (D-d.sub.v)/C.sub.A !(3)
wherein d.sub.v is the diameter of the vortex.
The above points may be shown as changes of the propagation time of the ultrasonic signal to time as shown in FIG. 2. Then, because the number of times of changes of propagation time of the ultrasonic signal per unit time is equal to the number of vortexes passing through the propagation path, i.e. to the number of produced Karman vortexes, the flow rate of the fluid may be found by counting the changes using a suitable device.
FIG. 3 depicts such a device for counting the mentioned changes, and comprises an electronic circuit 6, containing a pulse generator, amplifier and other components, an FM signal demodulator 7, a counter 8, etc.
Electronic circuit 6 applies a pulse signal to transmitter 4 which then transmits an ultrasonic signal to the vortex. Receiver 5 receives a signal which is modulated by the vortex in response to the ultrasonic signal from the transmitter 4, and then applies a pulse signal P.sub.o through electronic circuit 6.
Pulse signal P.sub.o has a frequency which is reverse-proportional to the total delay time, which corresponds to the sum of the delay time of transmitter 4, the propagation time within the fluid, the delay time of the receiver 5, etc, in the loop of the components.
Because the propagation time within the fluid changes each time the Karman vortex passes, pulse signal P.sub.o is a signal whose frequency is modulated by the vortexes. Pulse signal P.sub.o is demodulated by demodulator 7 and is applied to counter 8. The flow rate is obtained from the count obtained by counter 8.
However, although the vortex flowmeter described above is possible to achieve in theory, the following problem arises when attempting to realize the flowmeter in practice. The problem will be explained with reference to FIG. 4.
In FIG. 4, vortex generator 1 is disposed in the diametric direction of pipe 3, which is made of stainless steel, for example, and the transmitter 4 and receiver 5 are disposed on the outer wall of pipe 3 on the downstream side of the vortex generator 1 and facing each other. The transmitter 4 and receiver 5 are disposed so that they do not contact the fluid being measured.
Transmitter 4 transmits an ultrasonic signal B, shown by the dotted line, which is received by receiver 5, and which passes through the inside of pipe 3. Also, an ultrasonic signal C, which is a standing wave and is shown by the fine (see lower part) line, is received by receiver 5 after being repetitively reflected by the inner wall of pipe 3. In addition, an ultrasonic signal A, shown by the heavy line, is received by receiver 5 after passing through the propagation path crossing a vortex.
Ultrasonic signals B and C are noise signals which are detected by receiver 5 together with the ultrasonic signal A since these signals are transmitted by transmitter 4 as a continuous wave. Thus, in the case of FIG. 3, the vortexes cannot be detected with stability and without being adversely affected by noise.
The conventional vortex type flowmeter, as just described, cannot be realized in practice because of the noise propagated within the pipe and the noise resulting from the standing wave in the pipe. Also, in the flow meter of the type that detects changes of pressure on either side of a jetted fluid utilizing a piezoelectric sensor, such flowmeter cannot be used where the fluid is corrosive or is contaminated.